1. Field of the Invention
This invention relates to magnetic type article surveillance systems wherein articles to be protected from theft have affixed thereto targets made up of elongated strips of highly permeable, easily saturable magnetic material which, when carried past an interrogation antenna, cause a detectable characteristic magnetic field disturbance to activiate an alarm. More particularly, this invention in one aspect pertains to a novel method and apparatus for deactivating said targets so that when the protected article is purchased or accounted for, it can be brought past the interrogation antenna without activiating the alarm; and the invention in another aspect pertains to a novel method and apparatus for reactivating the targets.
2. Description of the Prior Art
Magnetic type article surveillances systems for protecting articles from theft are shown and described in French Pat. No. 763,681 and in the U.S. Pat. Nos. 4,118,693, 4,326,198 and 4,384,281. As described in those patents, the targets which are affixed to the protected articles are made up of thin elogated strips of highly permeable, easily saturable magnetic material such as permalloy. Also, as described in U.S. Pat. Nos. 3,747,086, 3,820,103 and 3,820,104, the targets can be made deactivatable by providing them with deactivation elements of a low permeability, magnetically hard material, such as vicalloy which can be magnetized to provide spaced apart north and south poles which are effective to break up the magnetic continuity of the target strips. The deactivation elements are magnetized and demagnetized by subjecting them to a powerful magnetic field generated by a deactivating and reactivating machine at a checkout or authorizing station.
A problem arises when the targets are used to protect magnetically sensitive materials such as recorded magnetic tapes, which are often mounted in cassettes. When a deactivating magnetic field is applied to a target mounted on the cassette, the field will extend inside the cassette and may damage the recording on the tape.
It has been proposed to provide an electromagnetic type deactivating device comprising a plurality of windings arranged on spaced apart core poles and a switchable electric circuit to cause direct current to flow through the windings and magnetize the poles in alternate directions when an object on which a target is mounted is positioned with the target extending along the ends of the poles. Such a device, which is shown and described in German Offenlegungsschrift DE No. 30 14 667 AL, is used with targets which are provided with a deactivation element in the form of a strip of hard magnetic material laminated along the length of the target. When the hard magnetic material is subjected to the alternate magnetic fields emanating from the poles it becomes semi-permanently magnetized in different directions at different locations along its length. The magnetized deactivation element subjects the target material to a corresponding magnetic bias at successive regions along its length. When the target material is so biased it cannot function to produce magnetic responses at harmonic frequencies when subjected to an alternating magnetic interrogation field. The semi-permanently magnetized deactivation element thereafter can be demagnetized in order to reactivate the target. The magnetic fields used to magnetize the deactivation element and the fields produced by the element when it is so magnetized are confined to a region in the immediate vicinity of the material and so do not affect the magnetic characteristics of nearby magnetic materials. Consequently this deactivating arrangement may be used with targets mounted to protect magnetic tape cassettes and similar products.
Problems have been encountered in connection with target deactivation devices as described above. These problems arise because the deactivation devices are quite bulky and expensive due to the multiple magnetic cores and associated windings. Moreover, the prior deactivation devices depend on complicated electrical circuits and switches which must be arranged to energize and deenergize the electrical circuits only when the object on which the target is mounted is properly positioned on the device. Finally, the electrical circuits in these prior deactivation devices are activated for a predetermined length of time following the closure of a switch when the targeted object is placed on the device. If the object is moved laterally during this time period, the pattern of magnetization which the magnetic cores had applied to the deactivation element on the target will be affected and this may result in residual activity in the target. That is, the target may not be completely deactivated.